The present invention is an improvement over the glass conveyor system disclosed in an earlier-filed application of the present inventor entitled Conveyor Drive Mechanism of a Glass Manufacturing System, U.S. Ser. No. 888,073, filed March 20, 1978 now U.S. Pat. No. 4,133,667.
In the system disclosed in the above-referenced application, a glass sheet was transported through a glass processing station on a roller bed formed of a plurality of elongated rollers. The rollers were supported on and in engagement with a continuous drive loop forming part of a conveyor drive. The continuous drive loop was trained over a pair of spaced pulleys, one pulley being disposed at each end of the glass processing station. By applying a driving torque to one of the pulleys of the conveyor drive, the continuous drive loop was caused to rotate and, in turn, impart rotational motion to the roller bed. The rotation of the roller bed caused the glass to be transported through the processing station.
The problem addressed by the invention disclosed in the earlier-filed application related to the phenomena of slip-stick friction and lost motion that were experienced when the conveyor drive was formed of a sprocket and chain arrangement. More specifically, the slip-stick friction phenomenon was caused by the chain experiencing a transition between the effects of high static friction and lower kinetic friction in a low speed range. In a transitional stage between these two types of friction, one portion of the chain can be experiencing high static friction while at the same time another portion is experiencing low kinetic friction. The chain between these two portions will stretch until the high static friction is suddenly overcome. The phenomenon manifests itself as an oscillation in the velocity of the moving chain and is disruptive to the transport of glass on the roller bed driven by the chain.
The lost motion phenomenon is particularly troublesome in an oscillating roller-hearth furnace that oscillates a glass sheet load within the furnace by alternatively driving the sprockets forward and backward. The lost motion is attributable to slack in the chain which must be taken out before rotation of the driving sprocket can be imparted to the roller bed.
The solution adapted by the earlier invention was to maintain a minimum predetermined amount of tension in the active portion of the continuous drive loop by the application of a counter-torque to the driven sprocket. One means of applying such a counter-torque was to use a pair of counter-poised motors with one motor connected in driving relation with each sprocket. Depending upon the amount of tension required in the continuous drive loop and the direction of travel of the glass, a control circuit selectively energized the pair of counter-poised motors in accordance with a predetermined control relation. Another means of applying the counter-torque to the driven sprocket in the case where the glass sheet travels continuously through this processing station was to connect the driven sprocket to a retarder. By either of these means, a minimum, predetermined tension was always present in the portion of the continuous drive loop in engagement with the roller bed.